1. Field of the Invention
This invention relates generally to air and water pollution abatememt and more particularly to a continuous, liquid phase, air oxidation process for converting sulfites and bisulfites present in waste water streams to non-polluting, non-oxygen demanding sulfates. The process of the invention is particularly suitable for treating sulfur recovery unit tail gas scrubbing liquors and sea water containing sulfite which has been used for scrubbing flue gas used for tanker inerting operations.
2. Statement of the Prior Art
In numerous industrial processes, notably in the petroleum and paper industries which produce waste streams containing sulfur compounds, problems of disposal frequently arise. These compounds are usually in the form of sulfides, polythionates or thiosulfates. Earlier methods of disposing of such compounds frequently consisted of limited treatment or simply dumping the untreated wastes, without regard to their Chemical or Biological Oxygen Demands, into receiving waters; or steam stripping and flaring hydrogen sulfide to produce SO.sub.2 which was released to the atmosphere. Such practices have been restricted around the world and various techniques are now being used to provide less polluting effluents. In some processes where sulfur is available as SO.sub.2 gas, as for example, in the tail gas of a sulfur recovery unit, the gases can be scrubbed with NaOH, NH.sub.4 OH or other suitable basic aqueous medium to remove the SO.sub.2. The present process provides an efficient way for treating the resulting (or "scrubbing") liquor which contains sulfite or bisulfite anions in association with sodium, ammonium or other cations.
In a related development, in the technique of inerting cargo tanks of oil tankers, flue gas from the ship's boilers is used to supply the inerting atmosphere. However, before the flue gas can be used for this purpose, it has to be cooled and scrubbed with seawater to remove SO.sub.2 and SO.sub.3 to minimize corrosion in the cargo tanks. Here the seawater contains sulfur predominantly in the sulfite form.
The conversion of sulfite and bisulfite ions to non-polluting sulfate ions does not require conditions as severe as does the oxidation of sulfides, polysulfides and polythionates. Examples of such severe conditions are found in coassigned U.S. Pat. No. 3,761,409 issued Sept. 26, 1973, which discloses a process for the air oxidation of sour water containing sulfides and intermediate sulfur compounds such as thiosulfates, tetrathionates, polythionates, sulfites and polysulfides to sulfate by adjusting the pH of the water to between about 6 to 13, heating the water to a temperature ranging from about 250.degree. to 520.degree. F; oxidizing the heated water under a pressure of around 75 to 800 psig at a liquid hourly space velocity of between 0.5 to 12.0 volume of feed per reactor void volume with a non-catalytic, oxidizing medium containing from 0 to 500% excess oxygen basis stoichiometric conversion of sulfide to sulfates and recovering an effluent containing substantially no other anion than sulfate.